1. Field of the Invention
The present invention relates to multi-beam scanning optical apparatus and image forming apparatus using it and, particularly, is suitable for image forming apparatus, for example, such as laser beam printers, digital copiers, etc. using the electrophotographic process, constructed to emit a plurality of beams from a light source means provided with a plurality of light emission points (light emitting regions), deflect the beams by a deflecting element, and guide the beams through a scanning optical element having the fxe2x88x92xcex8 characteristic to optically scan an area on a surface to be scanned, thereby recording image information.
2. Related Background Art
In the conventional scanning optical apparatus such as the laser beam printers (LBP), the digital copiers, etc., a plurality of beams optically modulated according to an image signal and emitted from the light source means are periodically deflected by an optical deflector, for example, comprised of a polygon mirror and are converged into a spot shape on a surface of a photosensitive recording medium by the scanning optical element (imaging optical system) having the fxe2x88x92xcex8 characteristic, to optically scan the surface to record the image.
With recent trends toward higher speed and higher resolution of the LBP bodies, dominating scanning optical devices are turning to multi-beam scanning optical devices for simultaneously scanning the area on the recording medium surface with a plurality of beams.
FIG. 6 is a schematic diagram to show the principal part of a conventional multi-beam scanning optical device. In FIG. 6 two diverging beams optically modulated according to image information and emitted from a multi-beam semiconductor laser 81 are limited in the size of their beam section by an aperture stop 82 and are converted into nearly parallel beams or converging beams by a collimator lens 83 to enter a cylindrical lens 84. The beams entering the cylindrical lens 84 emerge therefrom in the same state in the main scanning section. They are converged in the sub-scanning section to be focused as approximately linear images (linear images longitudinal in the main scanning direction) on a deflection facet 85a of the optical deflector 85. Then the two beams reflectively deflected by the deflection facet 85a of the optical deflector 85 are focused in the spot shape on the photosensitive drum surface 87 by the scanning optical element 86. With rotation of the optical deflector 85 in the direction of arrow A, the beams optically scan the area on the photosensitive drum surface 87 at an equal speed in the direction of arrow B (i.e., in the main scanning direction). This causes the image to be recorded on the photosensitive drum surface 87, which is a recording medium.
For adjusting the timing of scan start positions on the photosensitive drum surface 87 before the optical scanning on the photosensitive drum surface 87, part of the two beams reflectively deflected by the optical deflector 85 are guided through the scanning optical element 86 to a BD mirror 95, are reflected thereby to be condensed on a surface of BD slit 91, and thereafter are guided to a BD sensor 92. Then the timing of scan start positions for recording of image on the photosensitive drum surface 87 is adjusted using BD signals obtained by detecting output signals from the BD sensor 92. Each of the elements of the BD mirror 95, BD slit 91, BD sensor 92, etc. constitutes an element of synchronism detecting means (BD optical system). It is noted that FIG. 6 shows only one of the two beams.
In recent years, there are demands for compact layout of these scanning optical devices with trends toward smaller size of the LBP bodies. Among others, in the case of the multi-beam scanning optical apparatus for implementing simultaneous scanning with two beams, it is desirable that the BD optical system for detecting the write timing of scan start positions in the main scanning direction be complete only in a housing (optical box) to which the deflecting element and the scanning optical element are fixed, and thus there are demands for more compact optical systems.
For example, conceivable methods for realizing the compact BD optical systems include a method of using many return mirrors and bending BD optical paths thereby, a method of decreasing the focal length of the BD optical system in the main scanning direction and thus locating the synchronism detection position at a position extremely before the surface to be scanned, and so on.
However, the former method uses the many return mirrors and requires adjustment of tilt of the mirrors in the sub-scanning direction for guiding the beams to the BD sensor, posing the problem of high cost. On the other hand, the latter method decreases the image magnification in the main scanning direction between the light source and the BD slit at the synchronism detection position and it is thus difficult to spatially separate the spots of the two beams in the main scanning direction, for example, as illustrated in FIG. 7. Further, the decrease of the image magnification decreases the spot diameter itself, so as to narrow the depth, and thus the spot diameter tends to become larger because of manufacturing errors or the like. This tends to make the spatial separation of the spots more difficult.
If the spots of the two beams overlap at the synchronism detection position (i.e., at the position of the BD slit), a spot profile of one beam will be added to a spot profile of another beam originally to be detected, so as to degrade the accuracy of detection of synchronism in the main scanning direction, posing the problem that jitter appears in the image.
An object of the present invention is to provide a multi-beam scanning optical apparatus wherein a ratio of focal lengths of the scanning optical element and a synchronism-detecting optical element in the main scanning direction is set to an appropriate value, thereby permitting attainment of a compactification of the BD optical system and high accuracy of detection of synchronism based on prevention of spatial overlap between spots by an easy method, and to provide an image forming apparatus using it.
A further object of the present invention is to provide a multi-beam scanning optical apparatus wherein FNO (F number) of plural beams emitted from light source means is set to an appropriate value while keeping the focal length of the synchronism-detecting optical element shorter than the focal length of the scanning optical element in the main scanning direction, thereby spatially separating third dark rings of adjacent spots from each other at the synchronism detection position and permitting attainment of further improvement in the synchronism detection accuracy while maintaining compactness of the BD optical system, and to provide an image forming apparatus using it.
A multi-beam scanning optical apparatus according to one aspect of the present invention is a multi-beam scanning optical apparatus constructed so that a plurality of beams independently optically modulated and emitted from light source means having a plurality of light emission points placed with a spacing in a main scanning direction are focused in a spot shape on a surface to be scanned, via a deflecting element by a scanning optical element, the plurality of beams scan an area on the surface to be scanned, part of the plurality of beams via the deflecting element are converged in a spot shape at a synchronism detection position and thereafter guided onto a surface of a synchronism detecting element, and timing of scan start position on the surface to be scanned is controlled for each of the plurality of beams by use of a signal from the synchronism detecting element,
wherein a spot spacing in the main scanning direction between adjacent beams at the synchronism detection position is smaller than a spot spacing in the main scanning direction between adjacent beams on the surface to be scanned.
A multi-beam scanning optical apparatus according to another aspect of the invention is a multi-beam scanning optical apparatus comprising light source means having a plurality of light emission points which emit a plurality of beams capable of being optically modulated independently and which are placed with a spacing in a main scanning direction; a deflecting element for reflectively deflecting the plurality of beams emitted from the light source means, into the main scanning direction; a scanning optical element for focusing the plurality of beams deflected by the deflecting element, onto a surface to be scanned; and synchronism detecting means for guiding part of the plurality of beams deflected by the deflecting element, via a synchronism-detecting optical element onto a surface of a synchronism detecting element and controlling timing of scan start position on the surface to be scanned, by use of a signal from the synchronism detecting element,
wherein the following condition is satisfied:
0.02 (mm)/L less than fBD/fSC less than 1.0,
where L is a distance in the main scanning direction between adjacent light emission points of the light source means, fSC a focal length in the main scanning direction of the scanning optical element, and fBD a focal length in the main scanning direction of the synchronism-detecting optical element.
In the multi-beam scanning optical apparatus according to another aspect of the invention, said synchronism detecting means controls the timing of scan start position on said surface to be scanned, for each of the plurality of beams emitted from said light source means.
In the multi-beam scanning optical apparatus according to another aspect of the invention, said synchronism-detecting optical element is an anamorphic lens.
In the multi-beam scanning optical apparatus according to another aspect of the invention, said synchronism-detecting optical element is made of a plastic material.
In the multi-beam scanning optical apparatus according to another aspect of the invention, said scanning optical element is made of a plastic material.
In the multi-beam scanning optical apparatus according to another aspect of the invention, said synchronism-detecting optical element and at least one optical element forming said scanning optical element are integrally molded by plastic injection molding. In the multi-beam scanning optical apparatus according to another aspect of the invention, said synchronism-detecting optical element and said second optical element are integrally molded by plastic injection molding.
In the multi-beam scanning optical apparatus according to another aspect of the invention, said scanning optical element comprises a refracting optical element and a diffracting optical element.
In the multi-beam scanning optical apparatus according to another aspect of the invention, said synchronism detecting means is constructed in a plane of reflective deflection effected by said deflecting element.
An image forming apparatus according to one aspect of the invention is an image forming apparatus comprising the scanning optical apparatus as set forth, and a printer controller for converting code data supplied from an external device, into an image signal and entering the image signal into said scanning optical apparatus.
A multi-beam scanning optical apparatus according to a further aspect of the present invention is a multi-beam scanning optical apparatus comprising light source means having a plurality of light emission points which emit a plurality of beams capable of being optically modulated independently and which are placed with a spacing in a main scanning direction; a deflecting element for reflectively deflecting the plurality of beams emitted from the light source means, into the main scanning direction; a scanning optical element for focusing the plurality of beams deflected by the deflecting element, onto a surface to be scanned; and synchronism detecting means for guiding part of the plurality of beams deflected by the deflecting element, via a synchronism-detecting optical element onto a surface of a synchronism detecting element and controlling timing of scan start position on the surface to be scanned, by use of a signal from the synchronism detecting element,
wherein the following condition is satisfied:
fBD/fSC less than 1.0,
where fSC is a focal length in the main scanning direction of the scanning optical element and fBD a focal length in the main scanning direction of the synchronism-detecting optical element, and
wherein the following condition is also satisfied:
Fmi less than L/(6.656xc3x97xcex)xc3x97fBD/fSC,
where L is a distance in the main scanning direction between adjacent light emission points of the light source means, xcex an oscillation wavelength of the light source means, and Fmi an exit F number in the main scanning direction of the plurality of beams emitted from the light source means.
In the multi-beam scanning optical apparatus according to another aspect of the invention, said synchronism detecting means controls the timing of scan start position on said surface to be scanned, for each of the plurality of beams emitted from said light source means.
In the multi-beam scanning optical apparatus according to another aspect of the invention, said synchronism-detecting optical element is an anamorphic lens.
In the multi-beam scanning optical apparatus according to another aspect of the invention, said synchronism-detecting optical element is made of a plastic material.
In the multi-beam scanning optical apparatus according to another aspect of the invention, said scanning optical element is made of a plastic material.
In the multi-beam scanning optical apparatus according to another aspect of the invention, said synchronism-detecting optical element and at least one optical element forming said scanning optical element are integrally molded by plastic injection molding.
In the multi-beam scanning optical apparatus according to another aspect of the invention, said synchronism-detecting optical element and said second optical element are integrally molded by plastic injection molding.
In the multi-beam scanning optical apparatus according to another aspect of the invention, said scanning optical element comprises a refracting optical element and a diffracting optical element.
In the multi-beam scanning optical apparatus according to another aspect of the invention, said synchronism detecting means is constructed in a plane of reflective deflection effected by said deflecting element.
An image forming apparatus according to still another aspect of the invention is an image forming apparatus comprising the scanning optical apparatus as set forth, and a printer controller for converting code data supplied from an external device, into an image signal and entering the image signal into said scanning optical apparatus.